Optical transmission systems have come into widespread use primarily because of the ability of optical fibers to transmit much greater quantities of information than other comparable transmission media. Processing of such information normally requires that the information be converted to an electronic form. Thus, it has long been realized that if such functions as modulation, switching, mixing, data processing and the like could be performed directly on lightwaves, optical communications systems could be made to be much more efficient. It is also known that optically nonlinear materials can be used to make electrooptical modulators, switches, optical parametric devices and other devices for operating directly on lightwaves. Lithium niobate is the most commonly used nonlinear medium, although certain organic crystalline materials have also been proposed.
The patent of Dirk et al., U.S. Pat. No. 4,859,876, granted Aug. 22, 1989, hereby incorporated herein by reference, describes a nonlinear element comprising a glassy polymer containing an optically nonlinear organic moiety. The nonlinearity results from electric poling which aligns permanently dipoles within the polymer. The glassy polymer that was principally described was polymethylmethacrylate (PMMA), while other acrylate based polymers were also mentioned. The Dirk et al. patent represents a significant advance of the state of the art since polymers such as PMMA can be applied as a film to a substrate and their properties controlled much more easily and accurately than crystalline substances. The PMMA films constituting the heart of the various electrooptic devices may range from only about one micron to about two hundred microns in thickness.
A problem with the nonlinear devices of Dirk et al. is that their nonlinear susceptibility tends to deteriorate over time, particularly when subjected to high temperatures on the order of or exceeding 80.degree. C. Such lower susceptibilities generally mean that the devices perform the functions for which they were intended with less efficiency than would otherwise be the case. As a consequence, systems which use these devices may require special cooling apparatus to keep the devices from reaching elevated temperatures and other design precautions may be required to compensate for a deterioration of optical properties with time.